Part of Topical Section onFundamentals and Applications of Diamond and Nanocarbons

Influence of diamond-like carbonoverlay properties on refractive indexsensitivity of nano-coated optical fibres

Mateusz �Smietana*,1, Mariusz Dudek**,2, Marcin Koba***,1,3, and Bartosz Michalak****,1

1 Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75 00-662, Warsaw, Poland2 Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego 1/15 90-924, Lodz, Poland3National Institute of Telecommunications, Szachowa 1, 04-894 Warsaw, Poland

Received 11 April 2013, revised 10 May 2013, accepted 10 May 2013Published online 12 August 2013

Keywords diamond-like carbon, optical fibre sensors, optical properties, refractive index sensing, RF PECVD, thin film

*Corresponding author: m.smietana@elka.pw.edu.pl, Phone: þ48 22 234 6364, Fax: þ48 22 234 6063** e-mail mariusz.dudek@p.lodz.pl, Phone: þ48 42 631 3263 ext. 31, Fax: þ48 42 636 6790***e-mail m.koba@elka.pw.edu.pl, Phone: þ48 22 234 7246, Fax: þ48 22 234 6063****e-mail b.michalak@stud.elka.pw.edu.pl, Phone: þ48 22 234 6364, Fax: þ48 22 234 6063

An application of diamond-like carbon (DLC) nano-coatedoptical fibre as a promising platform for external mediumrefractive index (RI) sensing is investigated. The DLC thinfilms are deposited by radio frequency plasma enhancedchemical vapour deposition (RF PECVD) on fused-silica core(Ø ¼ 400 mm) of multimode polymer-clad silica (PCS) opticalfibres. The optical fibre sensor response to variations of theexternal RI for samples coated with DLC film for variousthickness and optical properties of the film is measured, andcompared to numerical simulations. We show that both opticalproperties and thickness of the DLC films depend on thedeposition time. The effect has a strong impact on the optical

response of the samples to variations of the external RI. Theresponse is mainly influenced by the thickness and RI ofthe DLC overlay. Moreover, the response also depends on theinvestigated wavelength range. We show that for intensity-based configuration at l � 490 nm, the DLC film thicknessshould reach about 80 nm. In such a configuration thesensitivity reaches over 2000% per RI unit (RIU). Forwavelength-based configuration, when shift of the intensitydeep at l � 490 nm is considered, the thickness of the filmshould be as high as 120 nm. In such a configuration thesensitivity reaches 250 nm RIU�1.

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction There is a great interest in applicationof optical fibre sensors, especially in places where traditionalelectronic or mechanical sensors cannot work due to harshenvironmental conditions [1]. One of the simplest opticalfibre sensing structure is based on short length (tens of mm)of core of the fibre exposed to external media [2]. In such aconfiguration evanescent wave interacts with the externalmedia resulting in modulation of the light intensity guided inthe fibre. In more advanced structures, the core is coatedby an overlay which typically absorbs or binds certainstimulant. The phenomenon induces variations of opticalproperties of the overlay, and thus modification in lightpropagation conditions in the optical structure [3]. There isanother group of sensors which are coated with an overlayshowing higher refractive index (RI, n) than the one of the

fibre core. For these sensors, the overlay at its certainthickness can modify the response of the optical fibrestructure to external RI. Such a sensing device offerssimplicity of fabrication and flexibility in determining itsfunctionality by proper selection of the applied properties ofthe overlay. Moreover, the device offers high sensitivity,which rivals some more complex optical structures [4]. Anumber of materials, including TiO2, In2O3, ITO and somepolymers, has been applied for discussed optical fibresensing configuration [4–6]. The overlays have beendeposited from liquid precursors by dip coating method.An application of vapour deposition methods for thispurpose which offer wide selection of materials andpossibility of in situ tuning of their properties is rare.In our previous works we have shown a capability for

Phys. Status Solidi A 210, No. 10, 2100–2105 (2013) / DOI 10.1002/pssa.201300059 a

statu

s

soli

di

www.pss-a.comph

ysi

ca

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

deposition of diamond-like carbon (DLC) overlay on opticalfibres [7]. DLC is a form of amorphous carbon orhydrogenated amorphous carbon exhibiting high hardnessand good resistance to chemical corrosion [8]. DLC istypically applied for passivation or protection of varioustools and devices [9]. DLC applied for electronic devices canplay a role of diffusion barrier against water molecules andsodium ions, two major sources of corrosion and instabilityin microelectronics [10]. In optical devices, DLC is typicallyapplied as both protecting and antireflecting coating [11].Some advanced optical sensors has also been demonstrated,where doped DLC has been used as a platform for RIsensing [12]. One of the biggest advantages of DLC iscomparing to e.g. diamond, its deposition at low temper-atures, which reduces the possibility of damaging substratematerial. In our works we applied radio frequency plasmaenhanced chemical vapour deposition (RF PECVD) methodfor deposition of these hard, smooth and wear resistant nano-overlays. It is also worth mentioning that DLC films showgood adhesion to fused silica glass, which additionallymakes them a good candidate for overlays enhancing sensingproperties of the optical fibre devices. Our up to dateinvestigations were based on measurements of powertransmission at a narrow wavelength range (l � 620 nm).We have already shown the capability of tuning the RIsensitivity by proper selection of the DLC overlayproperties [13]. Recently it has been shown by other authorsthat the sensitivity in such a sensing configuration highlydepends on the properties of the overlay, and what is more onwavelength range where the response is measured [14].

In this paper we discuss the influence of DLC filmproperties on the RI sensitivity in wide spectral range (l from350 to 1050 nm) in order to find optimal interrogationconditions for the sensor. Our experimental data aresupported by simulations based on developed numericalmodel. The simulations allow for further improvement ofthe device sensitivity, especially by proper selection ofDLC overlay properties.

2 Experimental details2.1 Modelling and simulations The numerical

model has been developed according to the fibre structurecross-section schematically shown in Fig. 1. The fibre hasbeen excited by a polychromatic light source at input

power Pin. The output spectrum Pout(l) is dependent on DLCnano-coating properties and optical properties of the externalmedium at the sensing region.

To determine the effective transmitted power (Pout) weemployed Eq. (1), which is commonly used for surfaceplasmon resonance sensors [15, 16]. This formula wasimproved in comparison to some other shown in, e.g. [5, 6,14], by taking into account not only meridional rays at angleof incidence in range from critical angle uc to p/2, where uc isexpressed by Eq. (2), but also skew rays in range from 0 toamax ¼ p=2 [15, 16]. At the maximum angle value, all themeridional and skew rays contribute to the Pout. In Eq. (1),N u;að Þ denotes the total number of reflection in the fibrecore in the region coated by DLC overlay, and is expressedby Eq. (3), where L is the length of the DLC region, D thecore diameter, a the skewness angle and u is the angle shownin Fig. 1. The Pin represents the power at the fibre input,which for wide light source can be represented according toEq. (4). It must be noted here that since in discussedcase er > 0, er > |ei| and er > eout, where er, ei and eout are,respectively, DLC’s real, imaginary and surroundingdielectric permittivity, then the film supports a lossy guidedmode, which exists both for transverse electric (TE) andmagnetic (TM) polarizations [17]. Finally, R in Eq. (1)represents averaged reflectivity of TM and TE modes givenin Eq. (5).

Pout ¼R amax

o

R p=2uc

RN u;að ÞPindudaR amax

o

R p=2uc

Pinduda; ð1Þ

uc ¼ sin�1 ncladdingncore

� �; ð2Þ

N u;að Þ ¼ L

D cosa tanu; ð3Þ

Pin / n20sinu cosu; ð4Þ

RN u;að Þ ¼ RN u;að ÞTM u;að Þ þ RN u;að Þ

TE u;að Þ2

: ð5Þ

In order to obtain reflectivity for TM and TE polarizedlight, we perform calculations based on the transfer matrixmethod (TMM) for the multilayer structure shown Fig. 2.

Figure 1 The cross-section of the investigated fibre sensorstructure.

Figure 2 Schematic multilayer model used to determine thereflectivity.

Phys. Status Solidi A 210, No. 10 (2013) 2101

www.pss-a.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

In our model the ith layer is described by the RI ni andthickness di. All the layers are assumed to be uniform andisotropic. The light inputs the first interface at an arbitraryangle u0. The tangential fields at the first boundary are relatedto those at the final boundary according to Eq. (6) where E0

and H0 are the tangential components of electric andmagnetic fields at the first interface, respectively, M is thecharacteristic matrix of the structure, and Em � 1 and Hm � 1

are the fields at the last interface.

E0

H0

� �¼ M

Em�1

Hm�1

� �; ð6Þ

M ¼Xm�1

i�1Mi ¼ M11 M12

M21 M22

� �: ð7Þ

In Eq. (7) theMi is described by Eq. (8), where bi and qifor TM and TE polarization and given by Eqs. (9), (10)and (11), respectively.

Mi ¼ cosbi     �isinbið Þ=qi�iqi sinbið Þ   cosbi

� �: ð8Þ

bi ¼2pdil

n2i � n20sin2u0

� �1=2; ð9Þ

qi ¼n2i � n20sin

2u0� �1=2

n2i; ð10Þ

qi ¼ n2i � n20sin2u0

� �1=2: ð11Þ

Finally, the reflectivity at wavelength l is given byEq. (12). This equation is used to obtain expressions for thereflectivity of TM and TE polarizations.

R u; lð Þ ¼ rj j2 ¼ M11 þM12qmð Þq0 � M21 þM22qmð ÞM11 þM12qmð Þq0 þ M21 þM22qmð Þ

��������2

:

ð12Þ

In our investigations we use three layers: core of the fibre(i ¼ 0), DLC nano-overlay (i ¼ 1), and external medium(i ¼ 2). The results of the reflectivity contributing tostructure’s transmission for various RI were normalizedusing data obtained for each sample in the air (nD ¼ 1).

2.2 Sensor fabrication In the experiment polymer-clad silica (PCS) multimode optical fibre (Øcore ¼ 400 mm)has been used. Outer polymer coating from 25 mm-longsegment of the fibre (total length of the fibre is 150 mm)was mechanically and chemically stripped [7]. The strippedsection of the silica core was cleaned in organic solventbefore the overlay deposition.

The RF PECVD setup used for DLC deposition isschematically shown in Fig. 3. The fibres were suspendedin a reactor chamber 5 mm over the RF electrode andreference oxidized silicon wafers were placed directly onthe electrode. The deposition parameters were optimizedfor good adhesion of the films to silicon dioxide (SiO2)substrates [18]. Each deposition was preceded by 5 min longion sputtering in RF plasma discharge at negative self-biasvoltage (VB) of 300 V and residual pressure of 13 Pa. Thedeposition processes were performed at 30 sccm methaneflow, pressure of 35 Pa and VB ¼ 200 V. The depositiontime range was from 1 to 13 min.

2.3 Thin film and sensor characterization DLCfilm properties were investigated on reference Si samplesusing Horiba–Jobin–Yvon UVISEL spectroscopic ellipso-meter (SE) in the wavelength range from 260 to 830 nm.The thickness and optical constants, i.e. n and extinctioncoefficient (k) were calculated by fitting of SE data usingsingle layer Tauc–Lorenc model of DLC film [18].

The transmission of the optical fibre samples wasmeasured in l range from 350 to 1050 nm using YokogawaAQ4305 white light source and Ocean Optics USB4000-VIS-NIR spectrometer. The applied measurement setup isschematically shown in Fig. 4. The nano-coated segment

Figure 3 Scheme of the RF PECVD setup applied for DLCdeposition on optical fibres and reference Si wafers.

Figure 4 Scheme of the experimental setup applied for external RImeasurements.

2102 M. �Smietana et al.: Influence of DLC overlay properties on RI sensitivity of nano-coated optical fibers

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

a ssp stat

us

solid

i a

was immersed in various liquids prepared by mixing waterand glycerine. The RI of the obtained liquid (nD) wasmeasured using Reichert AR200 Automatic Digital Refrac-tometer. Wavelength-dependent response of the structures tovariations in external RI was normalized using spectrumobtained for each sample surrounded by the air (nD ¼ 1).

3 Results and discussion The optical properties ofthe DLC film can be determined at the deposition stage byproper adjustment of a number of RF PECVD parameters,which include pressure in the chamber, gas flow andcomposition, self-bias voltage, temperature and depositiontime [18]. Evolution of the DLC film thickness and opticalproperties with RF PECVD time is shown in Fig. 5. Weobtained deposition rate reaching 11.8 nm min�1 andoptical properties of the film, which are highly dependenton deposition time. The obtained deposition rate allows forgood control of thickness of the deposited films. It is shownin Fig. 5b that the films reach the highest nwhen deposited ina 7 min long process. For process length up to 7 min, the nincreases with deposition time, whereas for longer processesit starts to decrease. A similar trend is observed for k wherefor 7 min long process k is the highest and the lowest in lrange below and above�420 nm, respectively. The influenceof deposition time on optical properties of the films has beendiscussed in details elsewhere [19]. The effect is based ondensification of the film in initial stage of its growth and then

release of hydrogen promoted by increase of temperatureduring the process. A significant influence of deposition timeon both DLC optical properties and thickness makes difficultindependent determination of these parameters.

It must be noted here that for the needs of theexperiment, the properties of the DLC films were determinedon reference Si wafers, not directly on the fibres. Due to thespecificity of PECVD method applied in our work, we mayexpect the films to be slightly different in properties on fibresand on reference Si wafers. However, microscopic images ofthe fibre cross-section (not shown here) confirm goodagreement between thickness of the film on the referencewafers and average thickness of the overlay on the fibres.Paying attention to this measurements and having in mind apossible imprecision in determination of the overlayproperties on the fibre, in the following discussion wemention properties of the overlays and refer more todeposition time and estimated thickness of the overlay thanprecisely defined thickness of the overlay on the fibre.

Taking into consideration dispersion curves of n and kobtained for 4 min long deposition, we simulated theinfluence of the DLC film thickness on sensor’s response toits immersion in water (nD ¼ 1.3330). It is shown in Fig. 6athat the maximum of transmitted power shifts towards longerwavelength with DLC thickness. The effect takes place in theinvestigated wavelength range up to 150 nm in filmthickness. Another maximum appears at DLC thicknessabove 250 nm, but it is lower in intensity than the one forthinner films, and it shifts less in wavelength with increase ofthe thickness. Depending on thickness of the DLC, thetransmitted power in selected wavelength ranges increasesby over 100% as a result of immersing the sample in water.Next, in order to investigate the influence of n and k onsensor’s response to immersing in water, we set film’sthickness to 80 nm, and modified n and k dispersion curvesby adding to them constant values. It is seen in Fig. 6b thatthe transmission maximum also shifts towards longerwavelength with increase of n. Simultaneously, it broadensits wavelength band and experiences reduction in intensity.The weakest effect to the sensors response has the increase ofk shown in Fig. 6c. A significant increase in k (by 0.3) resultsin slight shift of the transmission maximum towards longerwavelength and broadening of its band. Taking intoconsideration that for investigated DLC films k variationsare limited to 0.1, the effect resulting from its changes fromone film to another can be neglected. Proper selection ofoverlay’s n, and especially its thickness is critical from thepoint of view of the developed sensor performance.

Response of the DLC nano-coated samples to variationsin external RI from 1 to 1.444 has been investigated next. Theresults obtained for three of the most representative samplesare shown in Fig. 7. Depending mainly on DLC overlaythickness, the spectral response of the samples to changes inexternal RI is very different. In case of short depositions(<4 min, resulting in overlays below 50 nm in thickness)mainly a decrease in transmitted power is observed withincrease in external RI. For longer depositions, the

Figure 5 Evolution of DLC film (a) thickness and (b) opticalproperties, i.e. n and k, with deposition time.

Phys. Status Solidi A 210, No. 10 (2013) 2103

www.pss-a.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

transmitted power starts to increase in the lower wavelengthrange and shifts towards longer wavelength with RI.

When the thickness of the DLC overlay reaches about80 nm a dynamic increase in transmitted power with RI atl � 490 nm is observed. The sensitivity defined asvariations in normalized transmitted power per RI unit(RIU) reaches here 2000% RIU�1 and is linear in the whole

investigated RI range. For thicker DLC films (>100 nm) themaximum transmitted power further shifts towards longerwavelength, significantly losing its intensity and becominginsensitive to RI. But at shorter wavelength minimum oftransmitted power appears, which shift towards longerwavelength with RI. Sensitivity defined as shift inwavelength of the power minimum per RIU reaches here250 nm RIU�1. For samples with thicker DLC overlay, theobserved phenomenon is similar to the one noticed forsamples with a very thin film. It proves the periodicityobserved and discussed, in case of the results of simulationsfor the thickness variations.

Figure 6 Influence of the DLC (a) thickness, (b) n and (c) k ontransmission of the coated structure when external RI changes from1 to 1.3330. Simulations were performed for (a) experimental dataof n and k at 4 min long process. For (b) and (c) thickness was80 nm.

Figure 7 Influence of the external RI on transmission of the DLC-coated structures normalized to response recorded in air, where (a),(b) and (c) represent the effect for 4, 6.5 and 10 min long DLCdeposition time.

2104 M. �Smietana et al.: Influence of DLC overlay properties on RI sensitivity of nano-coated optical fibers

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

a ssp stat

us

solid

i a

4 Conclusions Besides typical application of DLC formechanical and chemical protection of various surfaces, thematerial can be successfully applied for tuning sensitivityof optical fibre devices. Presented approach is based onmultimode optical fibre with a 25-mm-long segment ofremoved cladding where the core is coated with the DLCnano-overlay. Relatively low-temperature process allows forDLC deposition on polymer-coated fibres. Thin, smooth andhigh-RI (n > 1.8) films were deposited on the sidewalls ofthe fibre using RF PECVD method. The properties of theDLC films can be easily modified by proper selection ofthe deposition process parameters. Applied numericalmodel, performed simulations and experimental dataconfirm the influence of thickness and optical propertiesof DLC on sensor’s RI response. Depending on choseninterrogation method, which can be based on variations intransmitted power at fixed wavelength or shift of thetransmitted power minimum in wavelength, a properthickness of the DLC should be deposited. For the DLCfilm thickness of about 80 nm, and at fixed l ¼ 490 nm,variations in the transmitted power with external RI reaches2000% RIU�1 where for thicker overlays (�120 nm in DLCthickness) the wavelength of the transmission minimumreaches the sensitivity of 250 nm RIU�1.

In contrast to other films applied for such devices, whichare typically deposited with dip coating methods, applied inthis work RF PECVDmethod allows for repeatable and timeefficient fabrication of the device. Its application is cheapwhen mass fabrication is considered. The platform seems tobe very promising for application in label-free bio-sensing.This approach allows for application of a chemically orbiologically sensitive film on top of the nano-coating,resulting in developing of both a highly sensitive andselective device.

Acknowledgements The authors gratefully acknowledgesupport for this work from the National Centre for Research andDevelopment of Poland within the LIDER program.

References

[1] F. T. S. Yo, and S. Yin (eds.), Fiber Optic Sensors (MarcelDekker Inc., New York, 2002).

[2] B. J.-C. Deboux, E. Lewis, P. J. Scully, and R. Edwards, J.Lightwave Technol. 13, 1407 (1995).

[3] A. P. Ferreira, M. M. Werneck, and R. M. Ribeiro, Biosens.Bioelectron. 16, 399 (2001).

[4] I. Del Villar, C. R. Zamarreño, M. Hernaez, F. J. Arregui, andI. R. Matias, Opt. Express 18, 20183 (2010).

[5] M. Hernáez, I. Del Villar, C. R. Zamarreño, F. J. Arregui, andI. R. Matias, Appl. Opt. 49, 3980 (2010).

[6] I. Del Villar, C. R. Zamarreno, P. Sanchez, M. Hernaez, C. F.Valdivielso, F. J. Arregui, and I. R. Matias, J. Opt. 12, 0955031 (2010).

[7] M. Smietana, J. Szmidt, M. Dudek, and P. Niedzielski, Diam.Relat. Mater. 13, 954 (2004).

[8] J. Robertson, Mater. Sci. Eng., R 37, 129 (2002).[9] A. Shirakura, M. Nakaya, Y. Koga, H. Kodama, T. Hasebe,

and T. Suzuki, Thin Solid Films 494, 84 (2006).[10] H. Voigt, F. Schitthelm, T. Lange, T. Kullick, and R. Ferretti,

Sens. Actuators B 44, 441 (1997).[11] M.H.Oliveira, Jr., D. S. Silva, A.D. S. Cortes,M.A.B.Namani,

and F. C. Marques, Diam. Relat. Mater. 18, 1028 (2009).[12] T. Tamulevi�cius, R. Šeperys, M. Andrulevi�cius, V. Kopus-

tinskas, Š. Meškinis, and S. Tamulevi�cius, Thin Solid Films519, 4082 (2011).

[13] M. �Smietana, F. Akoa Oyono, J. Grabarczyk, and J. Szmidt,Proc. Eng. 47, 1037 (2012).

[14] I. Del Villar, M. Hernaez, C. R. Zamarreño, P. Sánchez,C. Fernández-Valdivielso, F. J. Arregui, and I. R. Matias,Appl. Opt. 51, 4298 (2012).

[15] Y. S. Dwivedi, A. K. Sharma, and B. D. Gupta, Appl. Opt. 46,4563 (2007).

[16] D. Ciprian and P. Hlubina, Proc. SPIE 8306, 830612(2011).

[17] F. Yang, and J. R. Sambles, J. Mod. Opt. 44, 1153 (1997).[18] M. Smietana, W. J. Bock, J. Szmidt, and J. Grabarczyk, Diam.

Relat. Mater. 19, 1461 (2010).[19] M. Smietana, W. J. Bock, and J. Szmidt, Thin Solid Films

519, 6339 (2011).

Phys. Status Solidi A 210, No. 10 (2013) 2105

www.pss-a.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper